1. Field of the Invention
The present invention relates to a contact pin for a socket for electrical parts, and the socket for electrical parts, which is used for establishing an electrical connection between a semiconductor device (hereinlater called an IC package) or the like and a printed circuit board, by detachably holding the semiconductor device.
2. Related Art
As a conventional “socket for electrical parts” of this kind, there is provided, for example, an IC socket, as shown in FIGS. 10 to 12 that are described in Japan unexamined Laid-opened patent publication No. A-242977/1999.
The conventional IC socket 11, as shown in FIG. 10, is designed to be mounted on a printed circuit board 12 and to electrically connect an IC package 13 as “electrical parts” and the printed circuit board by holding the IC package 13 on the socket.
The IC socket 11 has a base plate 14, a tab film 15, a ball guide 16 and a pressing jig 17, in this order from the bottom. And these parts are detachably assembled by using a bolt 18 and a nut 19.
The base plate 14, as shown in FIG. 10, is formed to be a plate having a quadrangular shape, and at a peripheral portion of its 4 sides thereof, a plurality of round type pins 14a lines up in two lines. Inner side of the lines of the round pins, 4 bolt holes 14b, 4 positioning holes 14c are provided, and an elastic member 14d made of silicon rubber having elasticity is provided at a central portion of the base plate. The round pin 14a, as shown in FIGS. 10 and 11, passes through the base plate 14 and is provided with a fitting concave portion 14e at its upper portion. At a lower portion of the round pin 14a, an inserting portion 14f which protrudes downward from an under surface of the base plate 14 is provided and designed to be electrically connected to the printed circuit board by being inserted into a through hole 12a of the printed circuit board 12.
In addition, a tab film 15, as shown in FIGS. 10 and 11, is formed to be a thin sheet having a quadrangular shape and a size similar to the base plate 14. The tab film has, on its upper surface opposite to the IC package 13, an electrode pattern 15a for establishing connection with a set of arrangement of terminals 13a of the IC package 13 and has, on its lower surface opposite to the base plate 14, a pin type terminal 15b for establishing connection with the base plate 14. The tab film also has a conducting wire connecting the pin type terminal 15b and the electrode pattern 15a. 
The pin type terminal 15b is structured to be inserted into a fitting concave portion 14e of the round pin 14a of the base plate 14. In addition, the tab film 15 has also a bolt hole 15c and a positioning hole 15d having a size and at a position corresponding to the holes of the base plate 14 etc.
Further, the ball guide 16, as shown in FIGS. 10 and 11, is made of an electrical insulating material and has a quadrangular plate like shape having a size similar to a pressing jig body 20 of the pressing jig 17. At a central portion of the ball guide 16, a positioning opening 16a is provided for positioning a periphery of the solder ball 13a of the IC package 13. And at a position corresponding to the bolt hole 15c and the positioning hole 15d, a bolt hole 16b and a positioning hole 16c have a similar size to the corresponding bolt and the positioning holes (15c and 15d) of the tab film 15. The positioning opening 16a, as shown in FIG. 12, positions the balls 13a which are arranged at the outermost periphery of the matrix-like set of the solder ball arrangement.
Further, the pressing jig 17, as shown in FIG. 11, has the pressing jig body 20 having a quadrangular shape frame, a cover member 21 rotatably attached to the pressing jig body 20 through an axis 22, and a spring 29 urging the cover member 21 in an opening direction of the cover member 21 (clockwise in FIG. 11). A pusher member 23 for pressing the IC package 13 is attached through an axis 30 on the cover member 21 in a swinging manner. The pusher is urged by a spring 28 in a direction away from the cover member 21. Further, on the cover member 21 an engaged portion 24 is provided, and a latch member 25, which is designed to engage with the engaged portion 24, is provided through an axis 26 rotatably on the pressing jig body 20. The latch member 25 is urged clockwise in FIG. 12 (in an engaging direction) by the spring 27.
Into the pressing jig body 20, as shown in FIGS. 11 and 12, the IC package 13 having a quadrangular shape is designed to be inserted and be mounted on the tab film 15. From an under surface portion of the pressing jig body 20, a positioning pin 20a protrudes downward to be fitted into the positioning holes 14c, 15d and 16c. Further, in the pressing jig body 20, as shown in FIG. 12, a bolt hole 20b having the same size as the size of the bolt hole 16b of the ball guide 16 is provided at a place correspondent to the bolt hole 16b. 
The positioning pin 20a of the pressing jig body 20 is, as shown in FIG. 11, fitted into each positioning hole 14c, 15d and 16c of the base plate 14, the tab film 15 and the ball guide 16 so that each member is assembled in a predetermined positional relationship. And then into each bolt hole 14b, 15c, 16b and 20b of the base plate 14, the tab film 15, the ball guide 16 and the pressing jig body 20, the bolt 18 is inserted and screwed up together with the nut 19, to put all these parts on one another.
In such a socket mentioned above, when the IC package 13 is accommodated on the IC socket 11, as shown in FIG. 12, the outer periphery of the IC package 13 is guided to be positioned by an inner surface of the quadrangular frame-like pressing jig body 20 and also the solder ball 13a of the IC package 13 is guided and positioned by the ball guide 16.
An upper surface of the peripheral portion of the thus positioned IC package 13 is then pressed by the pusher member 23, as shown in FIG. 12.
However, in such a conventional socket, the fitting concave portion 14e of the round pin 14a of the base plate 14 is designed to be fitted to the pin type terminal 15b of the tab film 15 as an intermediate connector, being poor in workability of producing the round pin 14a having the fitting concave portion 14e and in workability of assembling and disassembling of the tab film 15. And as there is a need to bring the pin type terminal 15b into a fitting state, it is necessary to make the outer diameter of the round pin 14a larger, being inconvenient in realizing a socket having a narrower pitch of the round pins arrangement and consequently difficult in downsizing the IC socket 11.
In addition, as the IC package 13 is guided and positioned mainly by the pressing jig body 20, the pressing jig 17 must be replaced with another one so as to accommodate another IC package having a different size.
Further, the pusher member 23 is designed to press the IC package 13 in order to press the solder ball 13a on the tab film 15 so that when the pressing force becomes large, the solder ball 13a is excessively deformed in its shape.
There is another conventional IC socket other than the one mentioned above, which is designed to make contact between the under surface of the solder ball and the upper end portion of the contact pin. The contact pin, however, abuts strongly against the solder ball when the IC package is pushed with larger force, so that there is still a problem that the solder ball can be damaged.